Processes for the production of quality paper pulps, capable of obtaining, with most plants, suitable breaking lengths, are by nature essentially chemical in which the cellulosic fibres of the lignocellulosic plant material are freed from plant cement which binds them in the plants, consisting of hemicelluloses (sugar polymers with 5 to 6 carbons) and lignins (polymers of substituted allylphenols) by a chemical hydrolysis action in a concentrated basic or acidic aqueous medium, often in the presence of sulphur in different oxidation states.
These processes are at the present time employed essentially in existing industrial units throughout the world.
They have a major disadvantage in that they require considerable quantities (approximately 20% by weight) of inorganic chemical products during the cooking of the plants to make paper pulp. These inorganic chemical products are necessarily, but with difficulty, recycled and they are often the origin of foul odours due to the presence of sulphur.
Moreover, these factories require enormous investments in order to meet basically acceptable environmental standards, and they are therefore only profitable for a high critical size of the order of 100 to 200,000 tonnes of pulp produced per year.
A technological improvement has been obtained by replacing all or part of the water by an organic solvent of the alcohol, ketone or ester type, which makes it possible to overcome the use of sulphur, but not basic reagents, and therefore problems of recycling these reagents remain. These so-called “organosolve” technologies which require high pressures and involve high operating costs, are not as yet developed industrially for these reasons.
With the same idea in mind, other technologies of the same type using organic acids for hydrolysing hemicelluloses and lignins and at the same time for freeing cellulose fibres have been developed on the pilot plant scale. These technologies make it possible to do away with inorganic reagents completely, which is a considerable advantage.
Formic acid (B. BUCHOLZ and R K. JORDAN Pulp and Paper, p. 102-104, 1983; M N. ERISMANN et al., Bioresource Technology, Vol. 47, p. 247-256, 1994) can be used and it makes it possible to make acceptable paper pulps without pressure.
This technology also makes it possible to preserve in the paper pulps the silica contained in the plant, which is an important advantage when annual plants are used as a raw material since silica considerably disrupts the recovery of inorganic reagents in current industrial processes in a basic medium.
A variant of the process such as one under the name MILOX proceeds by cooking with formic acid in several stages in the presence of hydrogen peroxide, which improves delignification (K. POPPIUS-LEVLIN et al., Tappi Journal, Vol. 80, No. 9, p. 215-221, 1997).
Acetic acid can be used with the same aim but under pressure at a higher temperature (160 to 180° C.) at concentrations in water of 50 to 90% (R. A. YOUNG and J. L. DAVIS, Holzforschung, Vol. 40, p. 99-108, 1986).
Delignification is correct but the process requires washing of the pulp with acetone in order to remove lignins precipitated on the pulp.
A variant of this process makes it possible, with oxygen under pressure, to reduce the cooking time and to improve delignification (C. P. NETO and A. ROBERT, Holzforschung, Vol. 46, p 233-240, 1993) but it is at the origin of partial depolymerisation of cellulose by the joint action of pH and oxygen.
A variant of the MILOX process using acetic acid and hydrogen peroxide in two stages at 160-170° C. has also been proposed (K. POPPIUS-LEVLIN et al., Paper and Timber, Vol. 73, p. 154-158, 1991) but it does not provide any considerable improvement.
The limited acidity of acetic acid has led to its hydrolysis capacity being reinforced by the addition of hydrochloric acid (J. C. PAJARO et al., Holz als Roh-und Werkstoff, Vol. 54, p. 119-125, 1996) at 115-130° C. The reduction in the reaction temperature is the principal improvement of the process which has a major advantage of introducing chlorine ions into the process (G. VASQUES et al., Holzforschung, Vol. 49, No. 1, p. 69-73, 1995).
In addition, it should be pointed out that all the technologies using, in two or more stages, an organic acid and hydrogen peroxide generating peroxyacids in situ, are detailed in the review (N. LIEBERGOTT Pulp and Paper Canada, Vol. 97, No. 2, p. 45-48, 1996).
It should be added that technologies for bleaching these pulps without chlorine use hydrogen peroxide in a basic medium, which involves the regulation of silica in the form of sodium silicate causing considerable problems during the draining of pulps and the recycling of reagents.
The object of the present invention is to provide a novel process for producing paper pulp at atmospheric pressure from annual or perennial plants which leads to good-quality chemical pulps preserving the endogenous silica in their structure.
Document EP-A-0 584 675 teaches a process for extracting cellulose from lignocelluloses, by heating for two hours at high temperatures (170° C. or 180° C.) and under pressure in the presence of aqueous acetic acid with the addition of formic acid.
Document WO-A-95/21960 describes a process for cooking lignocellulosic materials, in particular from annual plants, with a mixture of carboxylic acids, involving a compulsory pyrolysis step.
The object of the invention is a process which makes it possible to obtain these performances whatever the nature of the plants used and which is thus particularly valuable in the case of annual plants in order to open up the way to new economic developments, in particular in the case of cereal straw and cane sugar bagasse or sugar sorghum bagasse.